Anti-Copy Protection for a Video Signal

ABSTRACT

An apparatus and method for applying anti-copy protection to a raster-scanned video signal are described. A square waveform  52  is added to the vertical synchronisation pulses  50  of the video signal. The effect of the waveform is not apparent until it has been recorded by a video cassette recorder. The anti-copy protected video signal can therefore be viewed normally on a display device at or near to the picture quality intended by the originator of the video signal, but once recorded, exhibits a reduction in picture quality which makes the video signal quite unattractive to view.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.10/909,918, entitled “ANTI-COPY PROTECTION FOR A VIDEO SIGNAL”, filed onAug. 2, 2004, which is a continuation-in-part of InternationalApplication No. PCT/GB2003/00450, filed Feb. 3, 2003, which claimspriority to GB 0202384.4 filed in the United Kingdom on Feb. 1, 2002,all of which are hereby incorporated by reference herein in theirentireties.

BACKGROUND OF THE INVENTION

The present application relates to a method and an apparatus formodifying a raster-scanned video signal such that the modified videosignal will still be viewable on a display device, such as a televisionscreen, but that following recording of the modified video signal,playback of the recorded modified signal will be adversely affected.

The present application provides a way of preventing unauthorisedcopying of an original video signal, and may be used, in particular, todeter copying onto tape of Pay-Per-View video signals, andvideo-to-video (tape-to-tape) copying.

Video piracy is a significant problem for broadcasters and distributorsof video signals, since the unauthorised copying and distribution ofvideo recordings by pirates can impact drastically on the revenuegenerated by a broadcaster or distributor through legitimate sales. Itis therefore desirable to prevent video pirates from making unauthorisedcopies of video signals.

This is best achieved by modifying the original video signal such thatwhen it is recorded the recorded signal cannot be satisfactorily playedback.

In known protection techniques, the unauthorised recording of the videosignal is made less enjoyable to watch by the interaction of theoriginal signal to which the protection has been applied with theelectronic components in either the video cassette recorder or thetelevision receiver itself. For example, making an unauthorised copy ofthe video signal too dark to be viewed satisfactorily may be achieved byadding to the original video signal a pulse which is significantlylarger than that part of the signal which carries the pictureinformation. The position at which the pulse is added depends on the wayin which the circuits in the television receiver or video recorderprocess the signal. When the modified signal is processed by theautomatic gain control circuits of a video cassette recorder, theamplitude of the signal is perceived as being that of the added pulseand not that of the portion of the signal carrying the usefulinformation. Consequently, the video cassette recorder or televisionreceiver amplifies the received signal by a smaller factor than if thepulse was not present. As a result of this the information-carryingportion of the signal is not therefore amplified enough to be seensatisfactorily when reproduced.

Such methods have however a number of drawbacks. Methods which rely onthe automatic gain control of the video cassette recorder, such asadding a large pulse to the signal, tend to result in a modified signalthat cannot itself be viewed on a television through the video channelregardless of whether the signal is being or has been recorded.

An alternative protection technique described in International PatentApplication WO 01/76240 involves the removal of a small number ofhorizontal synchronisation pulses from the blanking section of thesignal, so that an unauthorised recording of the signal cannot beproperly synchronised by the TV receiver on which it is to be playedback. As a result, the resulting picture playback can be poor.

A further technique, disclosed in International Patent Application WO96/31878, relies on inserting a pulse into the colour burst informationportion of the signal section of the signal, such that automatic gaincontrol circuits that rely on the average dc level of the colour burstto determine the necessary amplification of the signal, make suchamplification too small. An opposing pulse signal having a magnitudesufficient to offset the change in dc level of the colour burst portioncaused by the pulse signal, and optionally a second pulse, are insertedsomewhere from the last half of the remainder of the back porch of thesignal to the end of the start of the picture information portion.

We have found that this technique can be unreliable in practice, andfurthermore has the disadvantage that by inserting a pulse into thecolour burst part of the signal, the resulting picture quality isdetrimentally affected.

Both of these techniques rely on the components employed in the videocassette recorder or the television receiver, and, in some cases,certain video recorders or television receivers may have an arrangementof components that is not susceptible to the adverse picture effectscaused by the modified signal. Thus, the modified video signal can stillbe played without significant detriment to the picture playback and theanti-copy protection applied to the modified signal is rendered useless.

SUMMARY OF THE INVENTION

The invention is defined by the independent claims below to whichreference should now be made. Advantageous features are set forth in theappendant claims.

An apparatus and method for applying anti-copy protection to araster-scanned video signal embodying the invention are described inmore detail below with reference to the drawings. In a first aspect, asquare waveform is added to the vertical synchronisation pulses of thevideo signal. The effect of the waveform is not apparent until it hasbeen recorded by a video cassette recorder. The anti-copy protectedvideo signal can therefore be viewed normally on a display device at ornear to the picture quality intended by the originator of the videosignal, but once recorded, exhibits a reduction in picture quality whichmakes the video signal quite unattractive to view.

In a second aspect, a positive-going pulse is added to the back porch ofthe raster scanned video signal such that it is substantially contiguouswith the negative-going horizontal synchronisation pulse. The effect ofthe pulse is again apparent following recording of the video signal by avideo cassette recorder. A negative-going pulse 96 may be added afterthe colour burst 92 and a further positive-going pulse 98 at the startof the active line 84.

A circumvention apparatus and process are also provided to remove theprotection from an anti-copy protected signal.

Other aspects, features and details of the present invention can be morecompletely understood by reference to the following detailed descriptionof a preferred embodiment, taken in conjunction with the drawings andfrom the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention in a preferred embodiment will next be described indetail, by way of example, and with reference to the drawings in which:

FIG. 1 is an illustration of portions of a typical video picture signal;

FIG. 2 is an illustration of an example integrator circuit such as thatused to detect vertical synchronisation pulses in television receivers;

FIG. 3 is an illustration of the voltage output from the exampleintegrator circuit shown in FIG. 2, following the receipt of aconventional vertical synchronisation pulse;

FIG. 4 is an illustration of a modified signal in accordance with thepreferred embodiment of the invention;

FIGS. 5 a and 5 b illustrate a single modified vertical synchronisationpulse of the signal shown in FIG. 4 before and after recording;

FIG. 6 is a schematic circuit diagram showing a preferred set-top boxincorporating apparatus for producing the signal shown in FIGS. 4 and 5a;

FIG. 7 a is an illustration of a number of lines of a known unmodifiedpicture signal;

FIG. 7 b is an illustration of a modified PAL picture signal accordingto the second aspect of the invention;

FIG. 8 is a schematic circuit diagram showing the preferred embodimentof apparatus for generating the signal shown in FIG. 7 b; and

FIG. 9 is a schematic illustration of a circumvention device forremoving the protection from a signal protected according to the firstor second embodiments.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows part of a conventional raster-scanned video signal 2 for aPAL picture signal. Two blanking regions of the signal are shown with afew picture lines on either side. The upper blanking region shown is atthe end of an odd field and the lower one is at the end of an evenfield.

The picture signal is comprised of 625 lines 4 containing controlinformation 6, such as synchronisation pulses, which are used toconfigure the response of the television receiver to the signal andensure generation of a good quality picture, and usually pictureinformation 8. The line numbers 1-9, 308-320, and 622-625 are shown onthe figure.

The synchronisation pulses are used to reset the television receiver sothat it is ready to display the next line of picture information,following the completion of the previous line (horizontalsynchronisation pulses), or following completion of an entire field(vertical synchronisation pulses).

In FIG. 1, each picture information line 5, namely the lines at the leftside of the figure, can be seen to comprise picture information 8, shownas a positive going waveform, and a horizontal synchronisation pulse,10, shown as a negative-going pulse.

Following the lines containing picture information shown at the top-leftof FIG. 1, is the vertical blanking region 12 of the signal. This regioncontains the vertical synchronisation pulses 14 used to control fly-backfrom the bottom of the television screen to the top once a field orraster of the picture has been displayed.

In a PAL (or NTSC) signal, the picture is generated on the screen in tworasters that are interlaced with each other, and it is thereforenecessary to have two blanking regions. These are both illustrated forthe PAL signal in FIG. 1.

The first blanking region 12 begins at line 310 of the picture signaland the second blanking region 16 begins at line 623.5. Line 623 of thepicture signal therefore contains only a half line of pictureinformation. However, the television receiver is set up such that thisline lies outside of the viewable area of the television screen and,like the blanking section, is therefore not viewed. It will beappreciated by those skilled in the art that the lines of the picturesignal are numbered consecutively from 1 to 625, and that the signalbetween the two blanking regions is therefore continuous.

The blanking section itself comprises five negative-going equalisation18 pulses each of 2.3 .mu.s width, followed by five negative-goingvertical synchronisation pulses 14 each of 27.3 .mu.s width, followed bya further five equalisation pulses. The equalisation pulses 18 play asimilar role to the horizontal synchronisation pulses 10 and need not bediscussed further here as their function is well known to those skilledin the art.

The dc level 20, namely the voltage from which the line synchronisationpulses extend is called the blanking level. This voltage corresponds tothe colour black in the picture information.

Following the blanking region there are typically a number of lines 22of suppressed video information as shown in FIG. 1. These suppressedlines allow room for the transmission of information subsequent to thevideo information, such as that used in teletext systems.

As discussed above, the vertical synchronisation pulses control verticalfly-back between fields. A television receiver detects the presence ofthe vertical synchronisation pulses using an integrator circuit. Acapacitor in the integrator circuit charges, during each of thesynchronisation pulses, and discharges during the spaces in between. Thepositioning and the duration of the vertical synchronisation pulses issuch that the capacitor charges beyond a predetermined threshold thatwould not be reached due to charging from the narrower horizontalsynchronisation pulses or equalisation pulses alone.

FIGS. 2 and 3 respectively show an example of an integrator circuit andof the voltage across the capacitor as the synchronisation pulses arereceived.

The integrator circuit 30 shown comprises two input terminals 32 and twooutput terminals 34. As is well know in the art, a resistor 36 isconnected between first input and first output terminals, to form thefirst stage of the circuit, and a capacitor 38 is connected, after theresistor, across the output terminals to form the end-stage of thecircuit.

As horizontal or vertical synchronisation pulses are received at theinput terminals, the voltage across the capacitor increases as shown inFIG. 3. The duration of the horizontal synchronisation pulses andequalisation pulses produces only small increases in voltage as shown bypeaks 40 and 42 respectively. However the vertical synchronisationpulses produce a much greater increase in voltage, as illustrated bypeaks 44, such that the capacitor voltage exceeds a predetermineddetection threshold 46. The threshold need not be exceeded after thefirst synchronisation pulse, as shown in FIG. 3, but will, in normaloperation, be exceeded at least by the time the fifth verticalsynchronisation pulse is received.

First Aspect of the Invention

In accordance with the first aspect of the invention, a video signal ismodified such that it can be viewed on a television screen in the sameway as an unmodified signal, but following recording by a conventionalVideo Cassette Recorder (VCR), subsequent playback of the recordedsignal is unwatchable due to poor picture quality. The reduction in thepicture quality in particular appears as instability in the verticalhold of the picture causing it to jump and jitter on the screen.

This effect is achieved by adding an additional waveform into thevertical synchronisation pulses of the signal to interfere with theoperation described above with reference to FIGS. 2 and 3. As theblanking region is not used to contain picture information there is noloss in picture quality or data capacity of the signal beingtransmitted, such as might result if any of the active or suppressedpicture lines were modified.

The top half of FIG. 4, to which reference should now be made, showsvertical synchronisation pulses 50 modified according to the preferredembodiment of the invention. The bottom half of FIG. 4 shows anunmodified signal for comparison. The five negative pulses can be seento extend downwards from the blanking level 20, and can be seen to havean amplitude of about −300 mV.

The synchronisation pulses shown in the top half of FIG. 4 have beenmodulated according to the invention to include a waveform 52 with amuch shorter period than that of the vertical synchronisation period andwhich extends above the blanking level. It has been found that thisadditional waveform may have a frequency in the range 50 kHz to 5 MHz.Preferably, the waveform has a minimum voltage level of −300 mV, namelythat of the unmodulated vertical synchronization pulse, to a peakvoltage level of around +250 mV, that is above the blanking level. It isthus greater than the amplitude of the vertical synchronising pulsesthemselves. A waveform with a frequency of 1.1 MHz has been found toproduce the best results in practice, though 0.5 to 2 MHz is also anadvantageous range. The precise parameters that work best must be foundempirically for any particular application. In general, the lower thefrequency of the modulation, then the greater its magnitude needs to be,and vice versa.

The presence of the modulation or additional waveform 52 in the modifiedsignal does not have an effect on vertical synchronisation of thepicture until it is recorded. This is because the amplitude of theadditional waveform is too small before recording to have anysignificant effect on vertical synchronisation pulse detection. As aresult the modified signal may be viewed normally.

However, when the modified signal is recorded by a video recorder, thesignal is amplified. As will be appreciated by those skilled in the art,during recording the magnitude of the part of the signal lying below theblanking signal is ‘clamped’ or limited such that, even afteramplification, the amplitude of the negative-going horizontal andvertical synchronisation pulses remains substantially at thesynchronisation pulse level, that is the predetermined voltage requiredto ensure operation of the respective synchronisation pulse detectioncircuits.

As a result, the signal that is above the blanking level is effectivelyamplified in relation to the negative part of the signal and thereforetakes up a larger proportion of the total amplitude of the signal. Inthe case of the modified signal, this means that the positive part ofthe additional waveform is amplified in relation to the negative part.

This is illustrated in FIGS. 5 a and 5 b to which reference should nowbe made. FIG. 5 a shows one vertical synchronisation pulse 50 of asignal modified according to the preferred embodiment of the inventionto contain an additional waveform 52, before the signal has beenrecorded. The blanking level 20 is indicated on the figure, as is thezero-crossing point 54, or average value of the signal.

Preferably, the additional waveform is a square wave, as shown, as thisis found after amplification to have a greater effect on thezero-crossing point or average value than a substantially sinusoidalwaveform for example. The square wave does not necessarily have to havea duty cycle of 50%. Before recording the positive amplitude of thesignal might typically extend to +250 mV.

The amplitude of the additional waveform applied to the verticalsynchronisation pulse is not sufficient, before recording, to causeinstability in the vertical hold of the picture when viewed, eitherdirectly on the television or monitor, or indirectly through the videorecorder. However, when the signal is recorded, the part of theadditional waveform lying above the blanking signal is amplified inrelation to the negative-going synchronisation pulse, which remainsclamped at the same amplitude. This is illustrated in FIG. 5 b, whichshows the signal of FIG. 5 a after being recorded.

Depending on the type of video recorder, an initial modulated signal ofamplitude +250 mV might be amplified to +500 mV above the blankinglevel, while the signal below the blanking level remains clamped at anamplitude of −300 mV. As a result, the average level of the signal inthe region of the field synchronisation pulses is now less negative thanbefore it was recorded. The zero-crossing point or average value of thesignal in the region of the vertical synchronisation pulse is raised,and the capacitor does not charge as rapidly towards the predetermineddetection threshold. The television receiver cannot therefore detect thefield synchronisation pulses as easily, and the resulting recordedpicture, when played-back, will jump and jitter and not be enjoyable towatch.

Depending on the television receiver, the effect of the amplification ofthe modulation signal in the field synchronisation pulses might besufficient after just a single recording to result in the televisionreceiver having difficulty detecting the pulses. Nevertheless, each timethe modified video signal is re-recorded the amplitude of the modulationsignal is amplified in the positive direction, while the negativeamplitude of the synchronisation pulse remains clamped and constant. Thedisruptive effect of the modulation signal therefore becomes worse eachtime the modified signal is recorded, so that even if a firstunauthorised recording can be made, further recordings of the firstunauthorised recording are likely to be unwatchable.

The above description is a simplified explanation of the preferredsystem. It is not for example necessary for the upper part of theadditional waveform to be above the vertical blanking level for theprotection effect to be encountered. If for example a square wave isadded so that its minimum voltage level is below −300 mV, and itsmaximum voltage level is below zero, then the average voltage for theadditional waveform will be less than −150 mV. If the verticalsynchronisation pulse containing the additional waveform is thenrecorded, the synchronisation pulse and the waveform will be amplified.During recording, the minimum level of the pulse will then be clamped at−300 mV effectively discarding that part of the additional waveform witha lower voltage value. As a result, only the higher part of theadditional waveform will remain, thereby raising the average voltagelevel of the synchronisation pulse.

Furthermore, as most recorders boost the higher frequency part of thesignal more on recording than the lower frequency part, each time themodified signal is recorded, the high frequency modulation is amplifiedmore than the vertical synchronisation pulse in which it is disposed.The effect of the modulation is therefore made worse with eachsuccessive recording.

Referring again to the upper signal illustrated in FIG. 4, it can beseen that in the preferred embodiment of the signal a front porch 56 isadded to the signal before the first modulated vertical synchronisationpulse. The front porch shown has an amplitude of −300 mV, i.e. about thesame as that of the vertical synchronising pulses, and a width of 10.mu.s, though a range of 5 .mu.s to 15 .mu.s may be found to beeffective, and is found to improve the stability of the un-recordedmodified signal when viewed. Certain television receivers havedifficulty detecting the vertical synchronisation pulses when they havebeen modified according to the invention, and as a result jitter duringdisplay of the video signal on the television screen can occur evenbefore recording of the signal has occurred. The front porch thereforeacts like a single, unmodified vertical synchronisation pulse, and eventhough its duration is much shorter than typical verticalsynchronisation pulses, it has been found just enough to mark thebeginning of the blanking signal in which the vertical pulses arecontained and aid their detection by such television receivers.

The technique of vertical synchronisation pulse modification describedcan be used in conjunction with the line synchronisation pulse removaland modulation technique described in International Patent ApplicationWO 01/76240. It has been found that the section of the blanking regionin which the line synchronisation pulses have been removed, according tothe method disclosed in that patent application, is more frequentlymistaken by the television receiver as a vertical synchronisation pulsewhen the vertical synchronisation pulses are also modified in accordancewith this invention such that they are less easily recognised.

The above described techniques for producing an video signal withanti-copy protection have the advantage that the resulting modifiedsignal may be viewed on a television set through the video channel orotherwise, without any disruption to the picture quality. Only when thesignal has actually been recorded on a video recorder and is beingplayed-back is the effect on the picture quality realised. Thistechnique has considerable application to pay-per-view broadcasts, inwhich a broadcaster may, by transmitting a modified picture signal inaccordance with the preferred embodiment of the invention, prevent areceiver of the video signals from recording them onto tape. This allowsbroadcasting companies to broadcast programmes or movies before they aredue for release on video, and be sure that any subsequent licensed salesof the programmes or movies on video will be substantially unaffected.As the receiver of the programme cannot record it, the copyright in theprogramme may be protected.

The vertical synchronisation pulses of both PAL and NTSC signals can bemodified in the manner described above.

FIG. 6, to which reference should now be made shows a preferred circuitfor generating the modified video signal described above. This circuitmay be provided as a separate apparatus or, as is more likely, as partof a Pay-Per-View set-top box by cable or satellite broadcastingcompanies for home use. Other embodiments may also be possible however,such as an apparatus at a broadcaster's transmission site for applyingthe anti-copy protection before transmission.

The set-top-box 60 shown in FIG. 6 has a housing 62 in which inputterminal 64 is mounted. An unmodified video signal is received at theinput terminal 64 and is passed to an internal synchronisation generatorcircuit 66 and to video mixer 68. The synchronisation generator circuit66 strips the picture information from the received video signal andproduces a signal containing just the negative-going horizontal andvertical synchronisation pulses and the equalisation pulses. The‘stripped’ signal is then passed to a vertical synchronisation pulsedetector circuit 70 which detects the vertical synchronisation pulses ofthe signal and produces, as an output, a signal containing only verticalsynchronisation pulses of the required amplitude of −300 mV. The outputsignal is next passed to a square wave generator 72. The square wavegenerator 72 is triggered by the arrival of each verticalsynchronisation pulse to produce a square wave of predeterminedfrequency for the duration of the vertical synchronisation pulse. Thissquare wave is added to the vertical synchronisation pulse in order toproduce the desired modulation.

The output of the square wave generator 72 is the combination of thesquare wave and the vertical synchronisation pulse, which form themodified synchronisation pulse shown in FIG. 5 a. The modified signalfrom the square wave generator is then passed to the mixer 68.

Although a square wave generator has been described with reference tothe preferred embodiment, other types of waveform generator could alsobe employed.

Mixer 68 combines the signal containing the modified verticalsynchronisation pulses from the square wave generator 72 and theoriginal signal received from the input 64, such that the unmodifiedvertical synchronisation pulses of the original signal are replaced bythe modified vertical synchronisation pulses. The horizontalsynchronisation pulses and the original picture signal remaineffectively unaltered in the mixer.

The modified signal is then supplied to an amplifier 74, which amplifiesthe signal and supplies it to SCART connector 76 or a BNC connector (notshown), or to an RF output terminal 78.

The signal can then be viewed on a television or monitor in the usualway, but is rendered unwatchable when recorded by a VCR.

Although this aspect of the invention has been described with referenceto the PAL broadcast television standard, it may also be applied toother broadcast standard signals such as NTSC.

The above described technique cannot be directly used to protect videosignals that are to be recorded on video cassettes for distribution.This is because the recording of the original video signal onto anylegitimate copies will be subject to the same amplification of themodified vertical synchronisation pulses and any copies will thereforebe of a reduced picture quality.

Second Aspect of the Invention

A modified video signal in accordance with the preferred embodiment ofthe invention, which provides a protection method against video-to-videocopying, will next be described with reference to FIGS. 7 a, 7 b and 7c.

FIG. 7 a shows two adjacent active lines 80 of an unmodified videosignal 82. The video signal could be either a PAL or an NTSC signal.

Video information 84 is represented by the stepped waveform ascendingfrom the blanking or black level at its bottom to the white peak levelat its top. It will be appreciated that this waveform if viewed on atelevision screen would be seen as colour bars.

Between the active lines are negative-going horizontal synchronisationpulses 86 which control flyback in the television receiver betweensuccessive lines of the video signal. The horizontal region on the leftof a horizontal synchronisation pulse 36 is known as the front porch 88,while that on the right is known as the back porch 90. Colour burstinformation 92, used by the television receiver during demodulation ofthe chrominance part of the video signal is located on the back porch ofthe signal on each line and is represented purely diagrammatically by arhombus or diamond shape.

The video signal in FIG. 7 b has been modified according to theinvention in a preferred embodiment so that it is compatible with PALtelevision receivers. A signal suitable for NTSC television receivers isdescribed below in relation to FIG. 7 c.

The modified PAL signal includes a first additional pulse 94 with amagnitude that is approximately equal to the peak-white level; typicallyat a level of 1 to 1.2 volts. The first additional pulse issubstantially contiguous with the horizontal synchronisation pulse. Asshown it is situated directly adjacent to the horizontal linesynchronisation pulse. In fact the ascending or right-hand edge of thehorizontal synchronisation pulse meets the ascending or left-hand edgeof the additional pulse smoothly to form a continuous slope, that is tosay that there is substantially no space between the edges of the twopulses.

The presence of the pulse at this location has been found to have anadverse effect on the Automatic Gain Control (AGC) circuits of videorecorders. These circuits detect horizontal synchronisation pulses and,based on a determination of their amplitude, amplify the video signalsuch that it is suitable for recording. The combination of thenegative-going horizontal synchronisation pulse 36 and thepositive-going additional pulse 94 appears to the automatic gain controlcircuit as a larger than normal horizontal synchronisation pulse. As aresult the amplification provided by the automatic gain control circuitis less than it should be and is insufficient in respect of the rest ofthe video signal. The recorded signal, when played back later on atelevision receiver, will be too dark to be satisfactorily viewed.Furthermore it may exhibit stability problems, as the horizontalsynchronisation pulses and vertical synchronisation pulses have not beenamplified enough to be reliably detected by the circuits of thetelevision receiver.

As mentioned above, it is important for the implementation of thisfeature that the additional pulse be placed contiguous with thehorizontal synchronisation pulse. In particular, the pulse is not placedin the colour burst part of the signal, as this has been found to havean adverse effect on the picture quality while having little or noeffect on the automatic gain control circuits.

The amplitude of the pulse 94 is approximately white level, as shown,though it may be that an amplitude above 30% of white level will besufficient in certain circumstances. The duration of the pulse 94 isbetween 0.5 and 2 .mu.s and is preferably about 1 .mu.s. If the durationof the pulse 94 is at the larger end of the range, the horizontalsynchronising pulse 86 can be moved slightly earlier (to the left) toprovide sufficient space.

The presence of the first pulse 94 has however been found to have adetrimental effect on the playability of the modified signal even beforeunauthorised recording has occurred. This is because the pulseinterferes with the detection stage circuits in the television receivercausing the receiver to display the modified signal as a picture that isdarker than it should be. In order to compensate for this effect, asecond, negative going pulse 96, substantially equal in magnitude to anormal horizontal synchronisation pulse, is preferably added to thesignal directly after the colour burst. This additional negative-goingpulse has been found to reverse the ill-effect of first pulse 94 onlegitimate playback of the original modified signal. The duration ofpulse 96 is in the range 1 to 5 .mu.s, and is preferably 1.8 .mu.s.

The presence of second pulse 96 however also makes possible the additionof a third, positive-going pulse 98, located just prior to or at thestart of the active video information contained in the signal, anddirectly next to the negative-going pulse 96. The magnitude of the thirdpulse is about the same as that of the peak white level, and itsduration is in the range 1 to 4 .mu.s, preferably 2 .mu.s.

The third pulse acts in the same way as first pulse 94, by interferingwith the operation of the automatic gain control of the video recorder.The combination of the pulses 96 and 98 again appear like a horizontalsynchronisation pulse of larger magnitude than an ordinarysynchronisation pulse, and therefore interferes with the automatic gaincontrol circuits of the video recorder to add to the effect caused bythe first pulse 96. Without second pulse 96, the presence of third pulse98 is not enough to cause sufficient instability in the recorded signal.

Although in FIGS. 7 a and 7 b, only active lines of the video signalhave been shown, the pulse could be inserted throughout the entirety ofthe video signal, including the blanking section for example.

FIG. 7 c shows a video signal modified according to the invention in asecond aspect for use with NTSC television receivers. The signal issimilar to that shown in FIG. 7 b for PAL receivers except that thethird pulse 98 is missing and the second pulse is narrower and islocated before the colour burst rather than after it. In FIG. 7 c, thesecond pulse is given the reference numeral 99 to make it clear that itis different to the second pulse 96 in the case of PAL transmission. Thesecond pulse 99 has a duration in the range 0.5 to 1.2 .mu.s, and ispreferably 0.9 .mu.s. The horizontal synchronisation pulse 86 can bemoved slightly earlier if necessary to provide sufficient space for thesecond pulse 99.

The combination of first pulse 94 and second pulse 99, shown in FIG. 7c, has been found in the case of NTSC receivers to produce a signalthat, once recorded by a video cassette recorder, cannot be viewed on atelevision receiver, but that before recording can be viewed at or nearto the picture quality intended by the originator of the video signal.Once again, the presence of second pulse 99 is to undo the darkeningeffect that the pulse 94 has on playback of the television signal.

The two different signals illustrated in FIGS. 7 b and 7 c are requiredto implement the invention because of the different operation oftelevision receivers set up to receive PAL signals and NTSC signalsrespectively.

The preferred apparatus for adding pulses 94, 96 and 98 to the videosignal is shown in FIG. 8 to which reference should next be made.Preferably this apparatus is made use of by video distributors to addthe anti-copy protection described above to the video signal. Bothhowever include the first additional pulses 94.

The apparatus 100 comprises a housing 102 in which an external input 104is mounted for receiving an unmodified video signal. The signal ispassed from the input to a digital video processor 106 controlled bycontrol software 108. The digital processor analyses the receivedunmodified signal, and under the control of the software 108, addspulses to the signal in the manner described above. The digital videoprocessor outputs a signal containing the original signal and theadditional pulses to video amplifier 110. This amplifies the signal foroutput to a video output 112 external to the apparatus. The video outputthereby supplies a video signal that has been protected againstunauthorised copying according to the second aspect of the inventiondescribed above. The video distributor can then record this onto a videocassette using a professional video recorder in which the automatic gaincontrol circuit is turned off.

In addition to the protection methods described above, we haveappreciated that it is also desirable to provide a system and method ofdefeating or circumventing such protection. Thus, a protected signalcould be processed to remove the protection and give an unprotectedsignal that could be copied once again. A circumvention system 120 fordoing this is described in more detail in FIG. 9 to which referenceshould now be made.

A protected signal is first received at input 122. It is then passed toswitch 124 and to Horizontal and Vertical Synchronisation Pulsedetection circuit 126 respectively. The Synchronisation Pulse detectioncircuit 126 detects the presence of a vertical or a horizontal syncpulse, and sends a signal to controller 128. The presence of themodifications in the region of the vertical synchronisation pulse shouldnot affect the ability of the detection circuit 126, as at this stagethey will not have been recorded. In any case, the time constant of thedetection circuit 126 in the circumvention system is preferably smallerthan that of the corresponding circuitry in a television receiver. Inthis way, it may be possible to remove the vertical synchronisationpulse protection from signals which have been copied. The LM1881 chipmay for example be used as the Horizontal and Vertical SynchronisationPulse detector 126, with the SX20 chip used for the controller.

Depending on the output from the synchronisation pulse detection circuit126, the controller 128 operates the switch 124 to swap between firstand a second input. If no synchronisation pulse is detected, thecontroller 128 selects input 122 supplying the protected signal. As willbe recalled from the above discussion, the part of the protected signalaway from the synchronisation pulses is not modified in the protectionprocess and can be supplied to the output directly.

If a horizontal synchronisation pulse, or a vertical synchronisationpulse is detected however, the controller 128 selects the second inputof the switch, which is connected to signal generator 130. The signalgenerator 130 provides a signal to overwrite or replace that part of thesignal input at 122, which contains the modifications so that theprotection is no longer present. In the case of the modified verticalsynchronisation pulse, for example, the signal generator 130 preferablyprovides an unmodified vertical synchronisation pulse, whereas in thecase of the horizontal synchronisation pulse, the signal generatorpreferably provides an unmodified back porch. The timing applied to theswitch operation is therefore precisely controlled to ensure that onlythe previously modified areas of the signal are replaced. For thisreason, the signal generator receives an input from the controller 128ensuring that it is synchronised to the horizontal or verticalsynchronisation pulses detected by detector 126. In the case of themodifications to the horizontal line synchronisation pulse, this maymean swapping the switch back to the input signal at the appropriatetime to maintain the colour burst in the signal sent to the output.

As mentioned above, the signal generator may simply overwrite theprotection modified areas of the signal. In this case, the signalgenerator 130 may comprise a circuit that gives a controllable, constantoutput voltage, or a circuit that simply replaces the affected area ofthe signal with an unaffected area. In this way the verticalsynchronisation pulse, or the back porch region may simply be replaced.On the other hand, the removal of the modified areas may compriseaddition of a pulse or wave form to cancel that already in place in thesignal. In such a case, the signal generator may comprise a square wavegenerator or pulse generator.

Thus, reconstructing the unprotected signal may involving switching thesignal back to its original level at the appropriate time, replacing themodified signal with an unmodified signal, or adding a compensatingsignal.

The output from switch 124 is then passed to amplifier 132, and finallyto output 134. The AD8051 chip may be used as the amplifier.

Although the preferred apparatus and circumvention device have beendescribed with reference to a digital video processor and controlsoftware, it will be appreciated that both the control software and thedigital control software could be implemented in purpose-builtequivalent electronic circuits. Also, although reference has been madethroughout this application to a television receiver, it will beunderstood that any display device on which video signals can be viewedand which operates in a way equivalent to a television receiver todisplay a raster-scanned signal is included within the scope of theinvention. Furthermore, the operation of any apparatus described in thisapplication could be implemented partly or wholly in software asappropriate.

As is known in the art, both positive and negative transmission schemesexist, so called because of the polarity of the signal carrying part ofthe signal. The schemes described in the application above, by way ofexample, are positive transmission schemes, and the orientation of theadditional pulses as ‘positive’ or ‘negative’ reflect this terminology.It will however be appreciated that the invention also has applicationto negative transmission schemes. In this case, positive going pulseswill be negative going pulses, and negative going pulses will bepositive going pulses.

Although the present invention has been described with a certain degreeof particularity, it is understood the disclosure has been made by wayof example, and changes in detail or structure may be made withoutdeparting from the spirit of the invention as defined in the appendedclaims.

1. Apparatus for applying anti-copy-protection to a raster-scanned videosignal to produce a modified video signal, the apparatus comprising: aninput for receiving a video signal which is to be modified, the videosignal having vertical synchronisation pulses; circuitry configured tomodify the vertical synchronisation pulses of a video signal received bythe input to produce a modified video signal, the modified signal beingviewable on a display device at or near to the quality intended by theoriginator of the video signal but such that after recording by a videocassette recorder, the average voltage level of a modified verticalsynchronisation pulse is shifted towards the blanking level incomparison to the average level before recording, such that the recordedsignal exhibits a reduction in vertical stability when played back andviewed on the display device; and an output configured to output themodified video signal.
 2. Apparatus according to claim 1 wherein thecircuitry comprises a waveform generator for adding a waveform tovertical synchronisation pulses in the video signal received by theinput.
 3. Apparatus according to claim 2 wherein the waveform generatoris operable to add a waveform to the vertical synchronisation pulse withan amplitude greater than that of the vertical synchronising pulse. 4.Apparatus according to claim 2 wherein the wave form generator isoperable to add to the vertical synchronisation pulses a waveform thathas a peak level above the blanking level of the video signal. 5.Apparatus according to claim 2 wherein the waveform generator isoperable to add a square wave to the vertical synchronisation pulse. 6.Apparatus according to claim 2 wherein the waveform generator isoperable to add a waveform to the synchronisation pulse that has afrequency in the range 50 kHz to 5 MHz.
 7. Apparatus according to claim2 wherein the waveform generator is operable to add a waveform to thesynchronisation pulse that has a frequency in the range 500 kHz to 2MHz.
 8. Apparatus according to claim 1 wherein the modifying means isoperable to add a front porch to the video signal adjacent to the firstsynchronisation pulse.
 9. Apparatus according to claim 8 wherein thefront porch has a voltage level substantially equal to that of thevertical synchronisation pulse.
 10. Apparatus according to claim 8wherein the circuitry is configured to add a front porch to the videosignal that is between 5 μs and 15 μs in duration.
 11. Apparatus forapplying anti-copy-protection to a raster-scanned video signal toproduce a modified video signal, the apparatus comprising: an input forreceiving a video signal which is to be modified, the video signalhaving vertical synchronisation pulses; circuitry configured to modifythe only vertical synchronisation pulses of a video signal received bythe input to produce a modified video signal, the circuitry adding awaveform to the vertical synchronisation pulses with a frequency of atleast 5000KHz and with an amplitude at least equal to that of thevertical synchronisation pulses, and an output configured to output themodified video signal.
 12. A method for adding anti-copy-protection to araster-scanned video signal to produce a modified signal, comprising:receiving a video signal which is to be modified, the video signalhaving one or more vertical synchronisation pulses; modifying thevertical synchronisation pulses of the video signal such that themodified video signal is viewable on a display device at or near to thequality intended by the originator of the video signal, but such thatafter recording by a video cassette recorder, the average voltage levelof a modified vertical synchronisation pulse is shifted towards theblanking level in a comparison to the average level before recording,such that the recorded signal exhibits a reduction in vertical stabilitywhen played back and viewed on the display device; and outputting themodified signal.
 13. A method according to claim 12, wherein modifyingthe vertical synchronisation pulses comprises adding a waveform to atleast one vertical synchronisation pulse in the video signal received bythe input.
 14. A method according to claim 13, wherein the wave form hasa peak level above the blanking level of the video signal.
 15. A methodaccording to claim 13, wherein the waveform has a peak voltage level inthe range 0 to +250 mV.
 16. A method according to claim 13 wherein thewaveform is a square wave.
 17. A method according to claim 13 whereinthe waveform has a frequency in the range 50 kHz to 5 MHz.
 18. A methodaccording to claims to any of claim 12 wherein modifying the verticalsynchronisation pulses includes adding a front porch to the video signaladjacent to the first synchronisation pulse.
 19. A method according toclaim 18 wherein the front porch is between 5 μs and 15 μs in duration.20. A method for applying anti-copy-protection to a raster-scanned videosignal to produce a modified video signal, the apparatus comprising:receiving a video signal which is to be modified, the video signalhaving vertical synchronisation pulses; modifying only the verticalsynchronisation pulses of a video signal received by the input toproduce a modified video signal, the modifying means adding a waveformto the vertical synchronisation pulses with a frequency of at least500KHz and with an amplitude at least equal to that of the verticalsynchronisation pulses, and outputting the modified video signal.
 21. Araster-scanned video signal having modified vertical synchronisationpulses that the video signal can be viewed on a display device at ornear the quality intended by the originator of the video signal, butsuch that after recording by a video cassette recorder, the averagevoltage level of a modified vertical synchronisation pulse is shiftedtowards the blanking level in comparison to the average level beforerecording, such that the recorded signal exhibits a reduction invertical stability when played back and viewed on the display device.22. A raster scanned video signal having vertical synchronisationpulses, modified to include a waveform with a frequency of at least 500kHz with an amplitude at least equal to that of the verticalsynchronisation pulse, the waveform being disposed and only disposed inthe vertical synchronisation pulses.